The invention relates to a method for the production of a vacuum measuring cell with a diaphragm according to the preamble of patent claim 1 as well as to such a measuring cell according to the preamble of patent claim 18.
It is known to measure pressures or pressure differences by pressurizing a thin diaphragm and measuring its deflection. A known and suitable method for measuring the deflection of such diaphragm comprises implementing the diaphragm arrangement as a variable electric capacitor, wherein via measuring electronic circuitry in known manner the capacitance change, which correlates with the pressure change, is evaluated. The capacitor is implemented by disposing the thin flexible diaphragm surface at a short distance opposite a further surface and coating both opposing surfaces with an electrically conducting coating or implementing them of electrically conductive material. Due to the deflection, the distance between the two electrodes changes upon pressurization of the diaphragm leading to a capacitance change of the arrangement which can be evaluated. Sensors of this type are produced of silicon in large production numbers. The areal base body as well as also the diaphragm are herein often entirely comprised of silicon material. There are also designs with combined material composition, for example silicon with glass base. The sensors can thereby be produced cost-effectively. As a rule, pressure sensors of this type are only applicable for higher pressure ranges in the range of approximately 10−1 mbar to a few bar. High resolution at lower pressures from approximately 10−1 mbar on, are no longer realizable with the material silicon. Sensors of this type are not suitable for typical vacuum applications. The reason is inter alia that the silicon reacts on the surface with the environment and the sensitive sensor characteristic is thus disturbed. Water vapor contained in normal atmospheric air already leads to corresponding reactions on the surfaces. The problem is additionally exacerbated if the sensor is employed in chemically aggressive atmospheres, which in current reactive vacuum plasma processes is increasingly common. Attempts have therefore been made to protect such silicon sensors by passivating the surfaces against aggressive external actions. Attempts have also been made to provide the surface with a protective coating in order to increase the durability and resistance against the chemically aggressive environment, as has been described in DE 41 36 987. Such measures are difficult to realize and, in the case of mechanically deformable parts, such as diaphragms, only yield limited success, in particular with especially aggressive media, such as fluorine, bromic acid and their compounds, such as are utilized in the semiconductor industry, for example in vacuum etching methods.
It has therefore been proposed to produce measuring cells for vacuum pressure measurement of corrosion-resistant materials such as Al2O3. EP 1 070 239 B1 describes a capacitive vacuum measuring cell which is substantially completely built of ceramic and, consequently, is to a high degree corrosion-resistant. To be able to measure very low pressures up to 10−6 mbar with high accuracy, a very thin ceramic diaphragm of <250 μm thickness is utilized, which is disposed tension-free and symmetrically in a ceramic housing. Although this diaphragm-based vacuum measuring cell is very successful and represents a substantial advance with respect to corrosion resistance, it was found that the joining regions between diaphragm and housing, as well as the joining region for the connecting piece and, if applicable, the connecting piece itself, when used in aggressive process environments, which contain for example acids, halogens, such as chlorine and fluorine, etc., represent a weak spot regarding the service life of the cell even if the entire cell is substantially comprised of a corrosion-resistant ceramic. In the assembled state the measuring cell includes extremely small voids exposed to the process gases, which voids are offset in the form of a labyrinth and accessibility to the regions where the joining sites of the parts are located entails significant difficulty. Coverage of such regions in such small and difficult to access voids through suitable coating also appears scarcely possible, especially since during the coating the particles would have to be guided around the edges and corners of this labyrinth.